Alarm devices are required for many applications, including mobile construction, mining and industrial equipment. Such devices emit a sound in the audible range of human hearing to warn persons in the immediate area that an emergency or other unexpected event is taking place, or that a vehicle is backing up and that the operator of the vehicle may not see or be able to maneuver around those persons.
In the prior art of backup alarms, there is substantial technology for control of the volume of alarm sound emitted by such alarm devices. See, for example, U.S. Pat. Nos. 4,603,317 and 3,173,136. In addition, modern prior art electrically-operated backup alarms usually comply with criteria and standards promulgated by government agencies as well as self-regulating industrial organizations. See for example Section 1926.601 of the Department of Labor, Occupational Safety and Health Standards entitled "Motor Vehicles"; Department of Interior Mining Enforcement and Safety Administration Section 77.410; U.S. Army Corp of Engineers Manual Section EM385-1-1; and Society of Automotive Engineers (S.A.E.) Standard J994B.
Frequently, the sound level in typical prior art electrically-operated backup alarms is pre-set. The sound level in some alarms may be manually adjusted among more than one sound level using switches, shorting bars and the like. In still other types of backup alarms, the emitted sound level is automatically adjusted relative to sensed ambient noise levels. In such devices, the emitted sound level is automatically adjusted between pre-set maximum and minimum levels in accordance with recommended or required standards. The increase or decrease of emitted sound level between those limits is approximately linear and remains at a substantially constant level above the sensed ambient noise.
Most backup alarms are ruggedized, even hermetically sealed, to survive severe operating environments. Similarly, other alarm devices and other outdoor lighting and environmental sensor systems are also ruggedized and hermetically sealed.
In addition to concern for contributing to hearing impairment of persons working in or otherwise exposed to noise levels in excess of 62 db, operators of machinery simply tire of listening to loud alarm devices in their work place, especially when the machinery they are operating backup frequently. Since such alarm devices are important for safety and welfare of all personnel in the immediate areas of mobile construction equipment and industrial machinery, the risk of injury to such personnel and other persons in the area is increased if backup alarms are disabled or otherwise modified so they can not be heard above ambient noise levels. However, ambient noise levels vary and a simple, reliable and inexpensive method for setting the level of emitted sound from low-cost backup alarms is desirable.
In addition to simple, reliable and inexpensive adjustment between two levels of emitted sound levels from backup alarms for mobile construction, mining and industrial equipment, apparatus for simple, reliable and inexpensive adjustment of light intensity, sensor sensitivity or the volume or magnitude of the output of any other system powered from a DC source is desirable. Thus, a circuit for setting the output levels of a system having two pre-determined levels without a switch or other hardware, without separate power control, and, if sealed, without compromising the integrity of the seal is desirable.